Serotonin 5-HT1A receptor binding potential declines with age as measured by [11C]WAY-100635 and PET

Positron emission tomography (PET) and [11C]WAY-100635 were used to examine the effect of age on serotonin-1A (5-HT1A) receptor binding potential (BP) in 19 healthy subjects. Regions of interest (ROI) were drawn on the co-registered magnetic resonance imaging (MRI) in orbitofrontal (OFC), dorsolateral prefrontal (DLPFC), anterior cingulate (ACC), lateral (LTC), and mediotemporal (MTC), parietal, occipital and cerebellar cortex, and the raphe nuclei. BP values were calculated using a simplified reference tissue method. In addition, a voxelwise analysis was performed using SPM99. Voxelwise analysis revealed a significant global decrease of 5-HT(1A) BP with age (set level <.001). ROI analysis revealed significant age-related 5-HT(1A) BP decreases in DLPFC (r = -0.56), ACC (r = -0.44), OFC (r = -0.42), LTC (r = -0.40), parietal (r = -0.65), and occipital cortex (r = -0.43), but not in MTC or raphe nuclei. Overall, cortical 5-HT(1A) BP declined by approximately 10% per decade, except for the MTC, where we did not find a significant age effect. Hence, careful age matching may be recommended for future studies using PET and [11C]WAY-100635 to examine 5-HT1A receptors.

An introduction to PET and SPECT neuroreceptor quantification models

PET and SPECT using appropriate radioligands allow imaging of certain critical components of neurotransmission such as presynaptic transporters and postsynaptic receptors in living human brains. PET and SPECT data are commonly analyzed by applying tracer kinetic models. These modeling approaches assume a compartmental system and derive the outcome measure called the binding potential, which reflects the densities of transporters or receptors in a brain region of interest. New models are often noninvasive in that they do not require arterial blood sampling. In this review, the concept and principles of tracer kinetic modeling are introduced and commonly used PET and SPECT neuroreceptor quantification models are discussed.

Noradrenergic dysfunction in the prefrontal cortex in depression: an [15O] H2O PET study of the neuromodulatory effects of clonidine

BACKGROUND

Noradrenergic dysfunction has been consistently implicated in depression. Much of the evidence, though, has been indirect, such as an attenuated growth hormone response to the alpha2-adrenoceptor agonist clonidine. To more directly examine central functioning of the noradrenergic system in depression, we have used [15O] H2O positron emission tomography (PET) to measure cerebral blood flow (rCBF) in combination with clonidine as a neuromodulatory probe.

METHODS

Subjects were six depressed and six healthy women, medication free and matched for age and phase of menstrual cycle. Two PET scans were acquired at baseline and two scans at 20 and 35 min following an intravenous clonidine infusion of 1.4 microg/kg while subjects performed a sustained attention task.

RESULTS

The growth hormone response did not show a significant difference between groups. However, PET results revealed a difference in the right superior prefrontal cortex that was resolved as an interaction from decreased rCBF in healthy control subjects but increased rCBF in the depressed group, which was not accounted for by differences in task performance.

CONCLUSIONS

This differential effect of clonidine in the right prefrontal cortex provides in vivo evidence of noradrenergic dysfunction in depression, which we postulate arises from functionally impaired presynaptic alpha2-adrenoceptors as well as regionally "supersensitive" postsynaptic cortical alpha2-adrenoceptors.

Dilutions of low- vs. high-fat diets on intakes and gastric volumes in rats

Rats adapt to changes in dietary energy to maintain nearly constant energy intakes. This regulation indicates that animals sense and respond to nutrient content. We sought to determine whether this response was affected by the fat content of the diet. Our second goal was to determine how energy dilution affected intragastric volumes. Rats were randomized to high (18% w/w) and low fat (4.5% w/w) as the energy density of the diet was altered from 2.0 to 3.5 kcal/g. Average energy intake during 7-h feeds rose steeply (P<.01) when density was increased from 2.0 to 3.0 kcal/g, but modestly as density increased from 3.0 to 3.5 kcal/g. In other rats on 18% vs. 32% fat diets, energy intakes increased significantly (P<.01) as density of the diet was raised from 3.5 to 4.5 kcal/g. During diets at 2.0 and 2.5 kcal/g, animals on 18% fat ate fewer kilocalories than those on 4.5% fat; but over 3.0-4.5 kcal/g, energy intake was similar regardless of fat concentration (4.5-32%). Gastric contents after 7-h feeds increased with grams of food ingested similarly for high- and low-fat diets. We concluded that in rats: (a) compensation to energy dilution or concentration was inexact but (b) was about equal for high- vs. low-fat diets; thus, high fat was as well sensed as high carbohydrate; (c) compensations for energy densities were made despite varied gastric volumes; thus, rats learned to override the stimulus of gastric stretch and to sense energy via extra gastric mechanisms.

The effect of paroxetine on 5-HT(2A) receptors in depression: an [(18)F]setoperone PET imaging study

OBJECTIVE

In the cortex of animals, serotonin (5-HT) levels increase after several weeks of treatment with selective serotonin reuptake inhibitors (SSRIs). Studies using an intrasubject design to examine the effects of SSRI treatment on 5-HT(2A) receptors in the cortex of drug-free depressed patients are needed. In theory, agonist stimulation of 5-HT(2A) receptors could be relevant to SSRI treatment by promoting neuronal growth and survival as well as direct elevation of mood. The objective of this study was to evaluate the effect of 6 weeks of paroxetine treatment on 5-HT(2A) receptors in depressed patients.

METHOD

After a medication-free period of at least 3 months, 19 depressed patients were treated for 6 weeks with paroxetine, 20 mg/day. The authors used [(18)F]setoperone and positron emission tomography to assess 5-HT(2A) receptor binding potential in the patients before and after treatment and in 19 age-matched healthy subjects.

RESULTS

5-HT(2A) binding potential declined with age in all cortical regions in the depressed and healthy subjects. There was a significant interaction between age and treatment effect on 5-HT(2A) binding potential in all cortical regions. Subjects aged 20 to 30 years had a 10% decrease in 5-HT(2A) binding potential after treatment, whereas subjects aged 30 to 40 had no change. No regional differences in 5-HT(2A) binding potential between depressed and healthy subjects were found.

CONCLUSIONS

5-HT(2A) receptors down-regulate in young depressed subjects after treatment with paroxetine, but this down-regulation attenuates with age. This suggests that over 6 weeks paroxetine treatment increases 5-HT agonism on 5-HT(2A) receptors in the cortex of young patients with depression.

Modeling of receptor ligand data in PET and SPECT imaging: a review of major approaches

Over the past decade, a number of new kinetic modeling techniques have been developed for PET and SPECT ligands. This article will review commonly used modeling solutions for reversible positron-emission tomography (PET) and single photon emission computed tomography (SPECT) radioligands, with an emphasis on noninvasive methods. All of the modeling approaches in PET and SPECT assume a compartmental system and derive parameters that describe the compartmental system. These parameters will be defined, and their relationship to analogous parameters in pharmacology will be discussed. Then the major approaches are presented under the categories of graphical or mathematical as well as invasive or noninvasive.

Imaging the serotonin transporter with positron emission tomography: initial human studies with [11C]DAPP and [11C]DASB

Two novel radioligands, N,N-dimethyl-2-(2-amino-4-methoxyphenylthio) b enzylamine (DAPP) and (N,N-dimethyl-2-(2-amino-4-cyanophenylthio) benzylamine (D ASB), were radiolabeled with carbon-11 and evaluated as in vivo probes of the serotonin transporter (SERT) using positron emission tomography (PET). Both compounds are highly selective, with nanomolar affinity for the serotonin transporter and micromolar affinity for the dopamine and norepinephrine transporters. Six volunteers were imaged twice, once with each of the two radioligands. Both ligands displayed very good brain penetration and selective retention in regions rich in serotonin reuptake sites. Both had similar brain uptake and kinetics, but the cyano analogue, [11C]DASB, had a slightly higher brain penetration in all subjects. Plasma analysis revealed that both radiotracers were rapidly metabolized to give mainly hydrophilic species as determined by reverse-phase high-performance liquid chromatography. Inhibition of specific binding to the SERT was demonstrated in three additional subjects imaged with [11C]DASB following an oral dose of the selective serotonin reuptake blocker citalopram. These preliminary studies indicate that both these substituted phenylthiobenzylamines have highly suitable characteristics for probing the serotonin reuptake system with PET in humans.

A voxel-by-voxel analysis of [18F]setoperone PET data shows no substantial serotonin 5-HT(2A) receptor changes in schizophrenia

Several postmortem studies have reported regionally localized decreases in serotonin(2A) receptors (5-HT(2A)R) in schizophrenia. This was not confirmed by two recent [18F]setoperone positron emission tomography (PET) studies. In these two studies relatively large regions of interest (ROIs) were used; hence, 5-HT(2A)R changes may have been missed in some brain areas. Therefore, data from one study were analyzed on a voxel-by-voxel basis using Statistical Parametric Mapping (SPM). We also used this method to examine the relationship between 5-HT(2A)R binding potential (BP) and five PANSS-derived factors: negative, positive, activation, dysphoric and autistic preoccupation. Thirteen schizophrenic patients (10 antipsychotic-naïve, 3 antipsychotic-free; 11 M, 2 F; age 31+/-7 years) and 35 age-matched control subjects (15 M, 20 F; age 30+/-7 years) were scanned. The 5-HT(2A)R BP was determined for each voxel using the pseudoequilibrium ratio method on PET data obtained between 65 and 90 min after [18F]setoperone bolus injection. The resulting parametric 5-HT(2A)R BP images were spatially normalized using a ligand specific template. Analyses of covariance were done using SPM99 with age as covariate. In tests for the effect of schizophrenia and for partial correlations between 5-HT(2A)R BP and the five factors, corrected P values <0.05 at cluster or voxel level were considered significant. No significant differences were detected between patients and control subjects, and no significant correlations were observed between 5-HT(2A)R BP and any of the five factors. Thus, in agreement with the previous ROI studies, voxel-by-voxel analysis confirmed the lack of substantial 5-HT(2A)R BP differences between schizophrenic patients and control subjects.

beta-blocker binding to human 5-HT(1A) receptors in vivo and in vitro: implications for antidepressant therapy

A novel strategy for improving the treatment of depressive illness is augmentation of antidepressants with a 5-HT1(1A) autoreceptor antagonist. However, trials using the 5-HT1(1A)/beta-blocker pindolol are proving inconsistent. We report how positron emission tomography (PET) and in vitro autoradiography can inform trials of antidepressant augmentation. We show that in healthy volunteers, in vivo, pindolol (n = 10) and penbutolol (n = 4), but not tertatolol (n = 4) occupy the human 5-HT(1A) receptors, at clinical doses. Pindolol, as well as the beta-blockers penbutolol and tertatolol, has high affinity for human 5-HT(1A) receptors in post-mortem brain slices (n = 4). Pindolol shows preference for 5-HT(1A) autoreceptors versus the post-synaptic receptors both in vitro and in vivo. Our data reveal that pindolol doses used in antidepressant trials so far are suboptimal for significant occupancy at the 5-HT(1A) autoreceptor. Penbutolol or higher doses of pindolol are candidates for testing as antidepressant augmenting regimes in future clinical trials.

Novel radiotracers for imaging the serotonin transporter by positron emission tomography: synthesis, radiosynthesis, and in vitro and ex vivo evaluation of (11)C-labeled 2-(phenylthio)araalkylamines

A series of four 2-(phenylthio)araalkylamines have been radiolabeled with (11)C and evaluated as potential radiotracers for imaging the serotonin transporter (SERT) by positron emission tomography (PET). All four candidates display high affinity for SERT and low affinity for the dopamine or norepinephrine transporters using in vitro binding assays. Biodistribution studies in rats demonstrated that tail-vein injection of the (11)C-labeled radiotracers resulted in high brain uptake of radioactivity with a preferential distribution in brain regions known to be rich in SERT such as hypothalamus and thalamus. The most promising candidate, 16, had hypothalamus-to-cerebellum ratios of 9:1, 1 h postinjection, an indication of high specific to nonspecific binding. Ex vivo pharmacological studies demonstrated that uptake in SERT-rich brain regions was both saturable and selective for SERT. Two of the tested radiotracers, 15 and 16, have highly favorable properties for imaging SERT and will be used in pilot human PET imaging studies.

Variation in contrasting forms of 'memorising' and associated observables

BACKGROUND

There is a posited conceptual distinction in the student learning literature in higher education between contrasting forms of 'memorising': as a process of rehearsal which is usually equated with rote learning, and as a process of committing to memory material that in two separate senses is temporally either 'understood' before or after the event.

AIMS

The present study reports on the operationalisation of these contrasting forms of 'memorising' and investigates their empirical association with one another, their gender sensitivity, and their joint association with other modelling sources of explanatory variation in student learning.

SAMPLES

Two samples of entering first-year economics students at the Universities of South Australia (N = 896) and Adelaide (N = 448) which are combined in the present study (N = 1344). The combined sample is further distinguishable by gender (females, N = 662, males, N = 682).

METHOD

Students were surveyed prior to the commencement of lectures and reported retrospectively on their most recent school-based learning experiences. Resultant data in the form of inventory responses were subjected to exploratory and confirmatory factor analyses, multivariate analysis of variance, and correlation analysis.

RESULTS

Empirically the three forms of 'memorising' are independent of one another and they are furthermore sensitive to gender-based response differences in terms of both location and structure. Forms of 'memorising' are respectively associated in a theoretically congruent manner with 'deep'-level processes, learning pathologies, and contrasting conceptions of learning.

CONCLUSIONS

The unqualified use of 'memorising' in studies of student learning is contra indicated. Contrasting forms of 'memorising' represent discrete sources of explanatory variation that can be used to construct finer grained models of student learning in process terms.

Increased left posterior parietal-temporal cortex activation after D-fenfluramine in women with panic disorder

It is unclear whether the functional changes found in panic disorder reflect disturbed physiology of particular neurotransmitters. One method of investigating altered neurotransmission is to assess regional brain activations in response to agonist challenges. D-Fenfluramine is a medication that induces neuronal release of serotonin. Using ¿15OH(2)O and positron emission tomography (PET), measurements of regional cerebral blood flow (rCBF) were done at t=-20, -5, +20 and +35 relative to the IV D-fenfluramine injection (t=0) in nine panic-disordered and 18 healthy subjects. Subjects were otherwise healthy, right-handed, non-smoking and not taking psychotropic medication. ¿15OH(2)O PET scans were assessed with Statistical Parametric Mapping using individual global cerebral blood flow as a covariate. Comparisons of the (baseline) first two scans between healthy and panic-disordered subjects showed a decreased rCBF in the left posterior parietal-superior temporal cortex in the patient group. Fenfluramine-induced increases as defined by the last two scans minus the first two scans were compared between groups and a significantly greater increase in the same left posterior parietal-superior temporal region was found in panic-disordered subjects. Consistent with this finding, differences between the last two scans (post-fenfluramine) of the healthy and panic-disordered subjects showed an increased rCBF in the left superior temporal cortex in panic-disordered subjects. Functional pathology in the left parietal-superior temporal cortex in panic disorder may be related to abnormal modulation by serotonin.

Prefrontal cortex 5-HT2 receptors in depression: an [18F]setoperone PET imaging study

OBJECTIVE

Widespread disturbances of serotonin (5-HT) are implicated in the pathophysiology of depression. Of 5-HT receptor abnormalities reported, the most replicated finding is increased 5-HT2 receptor binding in the postmortem prefrontal cortex of depressed suicide victims. The extent to which these findings exist in depressed persons without recent suicide attempts is uncertain. The objective of this study was to evaluate 5-HT2 receptors in depressed patients who were medication-free and who had not made recent suicide attempts.

METHOD

With the use of [18F]setoperone and positron emission tomography (PET), 5-HT2 receptor binding potential was assessed in 14 depressed and 19 healthy subjects. Exclusion criteria for depressed patients included use of antidepressant medication within the past 6 months, a history of suicide attempts within the past 5 years, other current axis I disorders including bipolar disorder, and the presence of psychotic symptoms. The 5-HT2 (setoperone) binding potential in the two groups of subjects was compared by analysis of covariance with age as the covariate.

RESULTS

Age had a significant effect on 5-HT2 binding potential, but depression did not. The interaction of age and depression was not significant.

CONCLUSIONS

The 5-HT2 binding potential is not increased in untreated depressed subjects who have not made recent suicide attempts. This negative finding does not rule out the possibility that there is a role for 5-HT2 receptors in treatment or that 5-HT2 receptors are increased in highly suicidal states.

Gastric emptying of indigestible versus digestible oils and solid fats in normal humans

Recent scintigraphic studies indicate that lipolytic products in the small intestine do not inhibit gastric emptying of fat as potently as previously suggested by studies that compared a liquid indigestible oil with a solid digestible fat. The older studies left open the confounding possibility that solid fats emptied differently than liquid oil. We studied eight normal subjects who ingested four meals in which fat was (1) liquid, digestible Crisco oil, (2) liquid, indigestible sucrose polyester oil, (3) digestible, solid Crisco, and (4) indigestible, solid olestra. Fats were labeled with iodine-123, and their gastric emptying was followed with a gamma camera. Indigestible fats (whether liquid or solid) emptied consistently faster than digestible fats (P < 0.005), although differences were small. Solid fats emptied about as rapidly as oils in the first hour; but more slowly thereafter (P < 0.01). A comparison of present scintigraphic with older studies suggested that solid fats were not well tracked by duodenal, marker-perfusion techniques, which misled previous investigators.

The effects of single dose nefazodone and paroxetine upon 5-HT2A binding potential in humans using [18F]-setoperone PET

RATIONALE

Alterations in 5-HT2A receptor binding are implicated in suicidality and depression. 5-HT2A receptors may also be involved in the therapeutic effects of antidepressants.

OBJECTIVES

The purpose of this study was to assess the effect of paroxetine and nefazodone on 5-HT2A receptors after a single dose.

METHODS

Seven subjects received a single dose of nefazodone 200 mg and five subjects received a single dose of paroxetine 20 mg. Before and after the dose, 5-HT2A binding potentials (Bmax/Kd) were determined in each subject using [18F]-setoperone PET.

RESULTS

Nefazodone induced a significant change in 5-HT2A binding potential (-39+/-17%,, P = 0.003) while paroxetine showed no significant alteration of 5-HT2A binding potential (+3+/-13%, P = 0.73).

CONCLUSIONS

The change in 5-HT2A binding potential seen with nefazodone represents blockade of 5-HT2A receptors by the drug. We do not find evidence for acute downregulation of 5-HT2A receptors with paroxetine within 9 h.

Neurosteroid regulation of inhibitory synaptic transmission in the rat hippocampus in vitro

The effect of the neurosteroid dehydroepiandrosterone sulfate on inhibitory synaptic transmission was studied in area CA1 of the rat hippocampus using an in vitro hippocampal slice preparation. Synaptic responses elicited by stimulation of Schaffer collateral fibers were recorded extracellularly as population spikes in the somatic region and as synaptic field potentials in the dendritic region. Bath application of dehydroepiandrosterone sulfate (10 microM) enhanced the synaptically evoked somatic population spike with no effect on the dendritic synaptic potential. Isolation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor-mediated component of the synaptic response by addition of antagonists of N-methyl-D-aspartate and GABA receptors to the perfusion saline demonstrated that dehydroepiandrosterone sulfate had no effect on this component of the dendritic synaptic potential. In contrast, dehydroepiandrosterone sulfate antagonized GABA receptor-mediated inhibitory effects in the somatic region, resulting in an augmentation of the somatic population spike amplitude. Paired-pulse facilitation was unaltered by dehydroepiandrosterone sulfate, thus arguing against possible presynaptic sites of dehydroepiandrosterone sulfate's actions. These results indicate that dehydroepiandrosterone sulfate can alter synaptic transmission in the hippocampus through selective postsynaptic actions on inhibitory synaptic transmission. A synaptic effect of dehydroepiandrosterone sulfate is consistent with a neuromodulatory role for this neurosteroid in the central nervous system, and may contribute to the reported effects of dehydroepiandrosterone sulfate on cognitive processes such as learning and memory.

Variation in medical students' approaches to diagnosis: a basis for initiating conceptual change among teachers and students

OBJECTIVES

This study examines Finnish medical students' approaches to diagnosis and investigates further how medical teachers can use information about variation in their students' approaches to diagnosis to foster teaching in medical school.

DESIGN

The medical students responded to the Conceptions and Experiences of Diagnosis Inventory (CEDI).

SETTING

Faculty of Medicine, University of Helsinki.

SUBJECTS

Ninety medical students in their clinical years and eight clinical teachers from the same Faculty of Medicine.

RESULTS

The 11 subscales of the CEDI formed two contrasting factors: the first reflecting variation in 'non-virtuously' labelled, and the second in 'virtuously' labelled, aspects of diagnosis.

CONCLUSIONS

Cluster analyses revealed subgroup characteristics of students' diagnostic processes that are of potential benefit to both students and teachers. Teacher interviews indicated that, for students, the CEDI may act as a self-assessment tool to help develop their diagnostic and metacognitive skills. For teachers, the CEDI was seen to offer important information about their students' conceptions of diagnosis and diagnostic skills.

Assessment of spatial normalization of PET ligand images using ligand-specific templates

Recent advances allow robust computation of parametric maps of ligand-receptor binding from PET data sets. Parametric maps may be statistically analyzed at the voxel level, given suitable techniques for both the spatial normalization of image data into a standard space and the application of appropriate statistical tests. The purpose of this study was to spatially normalize parametric maps of [carbonyl-11C]WAY-100635 and [11C]raclopride binding using SPM 96 and ligand-specific templates. Ligand-specific templates were created from integral images taken from healthy subjects. For this, a MRI-based spatial normalization was used: T1-weighted MRI scans were coregistered to the PET integral images, and the spatial normalization of the MRI to the SPM 96 T1 MRI template was applied to the integral images. These integral images were meaned and smoothed to form [carbonyl-11C]WAY-100635 and [11C]raclopride templates. Reliability of spatial normalization using the ligand template method and the previous MRI-based spatial normalization was investigated by using a second set of integral images taken from a different cohort: Landmark coordinates were defined on all spatially normalized integral images. Mean coordinates were found in order to produce an overall (average) landmark for each location. For each image, at each location, the distance from the landmark coordinates to the overall landmark were found. A multivariate analysis of variance was used to examine the effects of observer variance, landmark location, and the method used. Visually acceptable templates were created. While observer variance was not significant, the landmark x method interaction was significant. The ligand template method had significantly smaller distances: Among the landmark locations with this method, the mean distances between individual image landmarks and overall image landmarks ranged from 1. 1 to 4.9 mm. The ligand template method provides a reliable approach for spatial normalization of PET ligand images.

PYY immunoneutralization does not alter lipid-induced inhibition of gastric emptying in rats

PYY is released from the distal ileum by fat and may be involved in mediating lipid-induced inhibition of gastric acid secretion and intestinal motility. The role of PYY in intestinal lipid-induced inhibition of gastric emptying in awake rats was investigated using a specific polyclonal antibody raised against PYY.

METHODS

Gastric emptying of liquids was measured in awake rats fitted with a Thomas gastric cannula. Intralipid (total dose 50 or 100 mg) was perfused for 10 min (0.05 ml/min) into a duodenal (n = 11) or mid-intestinal cannula (60 cm from Ligament of Treitz; n = 8), and gastric emptying was measured over the 5-10 min period. Gastric emptying was measured 15 min after IP injection of PYY (1 nmol/rat). PYY antibody (20 mg) or a control antibody (anti-KLH; keyhole limpet hemocyanin) was injected ip 8-12 h before experiments.

RESULTS

Exogenous PYY (1 nmol) inhibited gastric emptying and administration of PYY antibody blocked this response. Perfusion of lipid (50 and 100 mg) into the proximal intestine produced a 46% and 66% inhibition of gastric emptying respectively. Inhibition of gastric emptying in response to 50 mg lipid in the proximal small intestine was unaffected by administration of PYY antibody but was abolished by administration of the CCK A receptor antagonist devazepide (0.1 mg/kg ip). Perfusion of lipid into the distal intestine (50 and 100 mg) inhibited gastric emptying by 10% and 32% respectively. Inhibition of gastric emptying in response to 100 mg lipid in the distal intestine was unaffected by PYY antibody.

CONCLUSIONS

Lipid perfused into either the proximal or distal intestine inhibits gastric emptying via a PYY-independent mechanism. CCK is involved in proximal lipid induced inhibition of gastric emptying.

An exploratory student learning model of clinical diagnosis

This paper investigates how students vary in their conception and engagement of the diagnostic process. The research question is whether it is possible to isolate appropriate sources of variation that are sufficiently psychometrically and conceptually robust for modelling purposes. Eleven such sources of variation are reported, together with an associated common-factor empirical model that is readily interpretable in conceptual terms.

Length of intestinal contact on nutrient-driven satiety

Chemosensors throughout small bowel and colon inhibit food intakes when contacted by monomeric nutrients. We postulated that calorie-dependent inhibition of food intakes depended on additions of feedbacks from sensors in proximal and distal bowel contacted after high intakes of nutrients. Therefore, we determined how feedback from sensors in proximal gut interacted with feedback from simultaneously contacted sensors in distal bowel and whether suppression of nutrient intakes by intestinally perfused nutrients depended on length of gut contacted. Suppression of food intakes by maltose simply added to that from dodecanoate when both were present together either in proximal or distal small bowel. When dodecanoate was infused into proximal gut while maltose was infused distally, suppression of intake was threefold higher and was thus potentiated. Limiting contact of slowly absorbed lactose or oleate to 35 cm of jejunum nearly abolished the satiating potencies each exhibited during access to whole gut. The observations were consistent with our hypothesis.

Chemical specificities and intestinal distributions of nutrient-driven satiety

We measured intakes of sham- and naturally feeding rats during gut perfusions of nutrients. Our objectives were to determine 1) which nutrient products in gut lumen suppressed intakes; 2) how suppression by various nutrients is distributed along gut; and 3) whether time courses of suppression were similar among different nutrients. We found that satiating nutrients consisted of fatty acids only longer than 10 carbons, of monomeric carbohydrates only with affinity for the glucose transporter, and, among several amino acids, of only phenylalanine and tryptophan. Dimeric maltose had about the same potency as an isocaloric mixture of longer glucose polymers; since responses to either were blocked by a glucosidase inhibitor, each probably acted after hydrolysis to free glucose. Effective nutrients suppressed intakes about equally on infusion into duodenum vs. midgut, and the same nutrients also suppressed intakes when infused into colon. Food intakes were suppressed only while maltose was infused, not after it was stopped, but suppression persisted for 2 h after stopping perfusions with fatty or amino acids.

Role of small intestine in caloric compensations to oil premeals in rats

We postulated that dose-responsive satiety after oil premeals varies with the number of gut sensors stimulated by lipolytic products along intestine. These experiments in fasted rats on satiety after oil premeals were performed to 1) determine whether satiety was induced by lipolytic products but not triglycerides; 2) confirm that oil empties from the stomach at rates that vary with oil loads; 3) ascertain that increasing rates of oil entry into duodenum extend the length of gut contacted by lipolytic products; and 4) judge whether length of gut contacted correlated with dose-responsive satieties to dietary oils. 5) Using specific antagonists, we attempted to define how satiety was signalled by gut sensors. Timing and degrees of satiety did not correlate with timing and extent of gastric distensions but, rather, with the timing and extent of spread of lipolytic products along small bowel. Satiety after the highest premeal load of oil was blocked by Pluronic L-81, an inhibitor of intestinal secretion of apolipoprotein A-IV, but was unaffected by MK-329 (a specific antagonist of cholecystokinin) or by capsaicin blockade of chemosensory nerves.

No effect of depression on [(15)O]H2O PET response to intravenous d-fenfluramine

OBJECTIVE

Subnormal prolactin responses to the serotonin-releasing agonist fenfluramine occur in depression. Since many measures of serotonin pathology occur in depression, abnormal responses to fenfluramine may occur in brain structures other than the hypothalamic-pituitary axis. One study compared six depressed and six healthy subjects' responses to oral d,l-fenfluramine by assessing [18F]fluorodeoxyglucose uptake as detected by positron emission tomography (PET). That study showed several abnormalities within the cortex, and the authors concluded that low responsivity to d,l-fenfluramine is widespread in depression. In this study abnormalities in regional neuromodulation by serotonin in major depression were assessed with intravenous d-fenfluramine and [(15)O]H2O PET.

METHOD

Changes in regional cerebral blood flow (CBF) were detected by using [(15)O]H2O PET after administration of intravenous d-fenfluramine to 13 depressed and 18 healthy women. The PET scans were done 20 and 5 minutes before and 20 and 35 minutes after d-fenfluramine administration. Differences between the depressed and healthy groups in change in regional CBF (mean postfenfluramine minus mean prefenfluramine) were analyzed by using statistical parametric mapping.

RESULTS

There were no significant differences between depressed and healthy subjects; in fact, changes in regional CBF after intravenous d-fenfluramine were remarkably similar.

CONCLUSIONS

Degrees of neuronal responsivity to d-fenfluramine are similar in depressed and healthy subjects. Differences between the previous and current findings may be accounted for by greater specificity of intravenous d-fenfluramine to serotonin release, timing of scans, paucity of suicidal subjects in the current study, or greater variance in regional CBF from direct vascular effects of serotonin.

Inhibition of gastric emptying in response to intestinal lipid is dependent on chylomicron formation

Lipid in the intestine initiates feedback inhibition of proximal gastrointestinal function and food intake. In rats and humans, inhibition of gastric emptying is mediated, at least in part, by cholecystokinin (CCK)-A receptors, and in rats there is evidence for involvement of an intestinal vagal afferent pathway. The mechanism by which luminal lipid acts to release CCK or activate vagal afferent nerve terminals is unclear. The role of chylomicron formation in this sensory transduction pathway has been investigated using the hydrophobic surfactant Pluronic L-81 that inhibits chylomicron formation. Gastric emptying of liquids was measured in awake rats fitted with a Thomas gastric fistula and a duodenal cannula. Intestinal perfusion of lipid induced a dose-dependent inhibition of gastric emptying (6, 12, and 39% inhibition for 25, 50, and 100 mg lipid, respectively). Perfusion of lipid with Pluronic L-81 (2.8% wt/vol) reversed the lipid-induced inhibition of gastric emptying. Pluronic L-63, a chemically similar surfactant that has no effect on chylomicron formation, had no effect on lipid-induced inhibition of gastric emptying. Perfusion of the intestine with lipid (100 mg) increased plasma levels of CCK from 1.9 +/- 0.8 to 6. 5 +/- 1 pM. This increase was blocked by Pluronic L-81 but unaffected by L-63. These results provide evidence that chylomicron formation is important in the signaling of lipid in the intestinal lumen to CCK endocrine cells and to producing feedback inhibition of gastric emptying.